The use of ePTFE grafts and ePTFE stent/grafts for intraluminal repair is known in the art. Expanded polytetrafluoroethylene grafts as well as ePTFE stent/grafts, or covered stents, may be implanted in a radially compressed state generally using a catheter into a blood vessel, or virtually any body chamber in the body. The graft or expandable covered stent is typically positioned and released from a delivery catheter at a damaged area as desired. In the case of covered stents, a stent is often contained within an ePTFE graft, the stent providing outward pressure and support for the body lumen walls. The addition of the cover on the stent acts to reduce cell growth and occlusions in the interior of the lumen.
Grafts and covered expandable stents that are known in the art are disclosed in the following documents: U.S. Pat. No. 3,953,566 to Gore; U.S. Pat. No. 4,655,771 to Wallsten; U.S. Pat. No. 5,061,275 to Wallsten et al.; U.S. Pat. No. 5,112,900 to Buddenhagen et al.; U.S. Pat. No. 5,123,917 to Lee; U.S. Pat. No. 5,282,823 to Schwartz et al.; U.S. Pat. No. 5,282,824 to Gianturco; U.S. Pat. No. 4,850,999 to Plank; European Pat. Application No. 0 621 015 A1 to Lukic; European Pat. Application No. 0 551 179 A1 to Palmaz; DE 3918736 A1 to Vallbracht; Patent Cooperation Treaty Application WO 95/05131 to Gore, Patent Cooperation Treaty Application WO 95/05132 to Gore; Patent Cooperation Treaty Application WO 95/05555 to Gore; Patent Cooperation Treaty Application WO 87/04935 to Michelle. All documents cited herein, including the foregoing, are incorporated herein in their entireties for all purposes.
It is desirable, however, to provide a stent covering which expands and contracts in concert with an underlying stent. Some stents in particular undergo extreme axial elongation when radially compressed to a reduced diameter. When the diameter expands however, to its expanded state, the stent longitudinally shortens. Such a stent is accordingly loaded in a radially compressed and axially elongated state, and implanted by radially enlarging the stent to its implantation diameter.
Coverings of such stents are often insufficient as they fail to fully and completely flex and remain intact with a stent with exaggerated dimensions. A stent which shows such exaggerated axial elongation in accordance with radial shortening is shown in the above referenced Wallsten U.S. Pat. Nos. 4,655,771, and 5,061,275.
Expanded polytetrafluoroethylene is not an elastomeric material. It is therefore not in ePTFE's nature to return to an original state after it has been stretched. It is therefore difficult to use an ePTFE covering with such stents of exaggerated axial and radial variations as mentioned above because ePTFE is not able to stretch and recover in concert with the stent, for example PTFE is not readily plastically deformable. Methods of treating ePTFE have been developed, however, in order to enhance ePTFE's physical expansion and recovery characteristics.
For example, U.S. Pat. No. 4,877,661 to House et al. discloses an ePTFE which is formed by extruding, compressing, heating, cooling and then stretching it back to its original length. The microstructure of the porous ePTFE material consists of nodes interconnected by fibrils; substantially all the fibrils having a bent or wavy appearance. The bent structure allegedly provides the ePTFE with properties of “rapid recovery”; i.e. when the ePTFE tube is pulled, the fibrils then have a tendency to return to the bent state.
U.S. Pat. No. 6,039,755 to Edwin et al. discloses an ePTFE tube which is used as an implant. The tube is implanted and radially expanded in vivo, and such radial expansion deforms the ePTFE material by elongating its nodes past the elastic deformation of the ePTFE.
U.S. Pat. No. 5,788,626 to Thompson discloses an expandable stent/graft with an ePTFE cover, the ePTFE cover having a bi-axially oriented node-fibril structure with folded fibrils.
U.S. Pat. No. 4,830,862 to Yamamoto et al. discloses a heat shrinkable tetrafluoroethylene polymer tube which is radially expanded, and serves to make a tube which will heat shrink around another article to form a composite article with a tetrafluoroethylene cover heat-shrunk thereto.
While the above referenced patents attempt to address the need for an ePTFE composition with recovery properties, they fall short in providing an ePTFE covering capable of stretching and recovering in concert with a stent having extreme radial expansion and axial elongation properties, such as those described stents in the Wallsten patents listed above. There is a need for an ePTFE material which has the capability of dimensional changes in the axial and radial direction, without substantial plastic deformation of the material or without substantially changing the fibril length. The present invention is therefore directed to overcoming the drawbacks and deficiencies of the prior art.